Enable diffing against any commit.

Closes #309.

doc/gitgutter.txt

@@ -117,6 +117,7 @@ You can customise:
 - The sign column's colours
 - The signs' colours and symbols
 - Line highlights
+- The base of the diff
 - Extra arguments for git-diff
 - Key mappings
 - The grep executable used
@@ -185,6 +186,14 @@ colorscheme or |vimrc|:
   GitGutterChangeDeleteLine " default: links to GitGutterChangeLineDefault
 <
 
+THE BASE OF THE DIFF
+
+By default buffers are diffed against the index.  To diff against a commit
+instead:
+>
+  let g:gitgutter_diff_base = '<commit SHA>'
+<
+
 EXTRA ARGUMENTS FOR GIT-DIFF
 
 To pass extra arguments to git-diff, add this to your |vimrc|: